CS 1.6 lag with a lot of players

Well I have a big craze of counter strike.
When I join a gun game server its ok in the beginning regardless of the no. players. After a minute or two the game becomes slower , slower and slower. But when a round is over, in case any side wins then when the next round starts it becomes normal. But again it becomes slower as the the time progresses.

Since in gungame server it is very unlikely that a round would end but sometimes it do happen so I cant play properly on it.